Male gelada monkeys—such as the one shown here with two smaller females—often make the unusual call, which University of Michigan researcher Thore Bergman says could represent an evolutionary precursor to speech.

Talkin' smack: Babbling monkeys may be 'missing link' for how speech evolved

The sound was a low, rolling chatter, like the din of a cocktail party. But Thore Bergman wasn’t at a party. He was high atop a grassy plateau in the Simien Mountains of Ethiopia. The behavioral biologist held in his hand not a champagne flute, but a digital recorder. And the hundreds of chatty creatures that surrounded him weren’t men in monkey suits—they were monkeys.

In more than a decade spent studying monkeys, Bergman had never heard that sound. “It was a strange sensation,” recalls the University of Michigan researcher. “You sort of looked over your shoulder to see who was talking to you, but there was no one there.” The similarity was more than a mind trick. Gelada monkeys’ melodies closely mirror the rhythms of human speech, making them a potential clue as to how speech evolved in humans, Bergman reports this week in the journal Current Biology.

“As far as we know there are no other examples of this,” he says.

A moaning, groaning, yawning lot, the vocal repertoires of geladas have earned them a reputation as the Adeles of the monkey world. But the sound that perked up Bergman’s ears in 2006 was different. The geladas smacked their lips together, a common facial gesture in baboons, which are their close relatives. However, unlike baboons, the geladas did so while vibrating their vocal chords, producing a rhythmic hum. Bergman called this hum a “wobble.”

That wobble “is the best evidence for the theory that speech evolved from facial expression,” says Asif Ghazanfar, a primate neuroscientist at Princeton University whose research spurred Bergman’s new finding. “It’s in essence a primitive form of babbling. And the rest of human speech is basically just elaborating on that babbling,” he says. Since the evolution of speech is not contained in the fossil record, the theory can’t be proven. But both researchers agree it’s a promising link.

Ghazanfar has reason to be enthused. Last year, he published a paper in Current Biology which analyzed lip-smacking patterns in macaque monkeys. He X-rayed the monkeys’ skulls to find that their silent smacking was mechanically and rhythmically similar to humans talking (and distinct from chewing). This facial expression, Ghazanfar wrote, could be a precursor to speech.

Just one thing was missing from his study: to speak, you have to make noise. Until then, lip-smacking monkeys were only known to open and close their mouths in silence. “How can lip-smacking be related to speech if there is no vocal component?” the paper asked.

That was a question Bergman could answer. When he came across the paper, he immediately e-mailed Ghazanfar to tell him what he had observed in geladas. This was Ghazanfar's missing link. “I was really excited about it," he says. “Send me the video so I can put it in my presentation," he recalls writing back.

As Bergman leafed through the literature, he realized that no one had yet described the wobble he had found. Next, he analyzed the sounds digitally, scoring the intervals between each sonic peak. Bingo. The wobbles fell into the same rhythmic range—three to eight cycles per second—as human speech. He now had a potential piece of the “evolutionary puzzle” of how speech formed.

Still puzzling is the sense behind the sound. In the wild, it is usually male geladas who make the wobble as a friendly gesture toward females. "It's kind of an appeasement call," says Colleen McCann, who is the mammal curator at New York's City's Bronx Zoo and has collaborated with Bergman. Similarly, macaque monkeys and marmosets take turns lip-smacking as a sign of friendly bonding. But other baboons mainly converse in the form of staccato grunts.

So why did geladas need to learn new coos?

Bergman, who co-directs the University of Michigan Gelada Research Project with his wife, behavioral ecologist Jacinta Beehner, has a theory. First, more calls may have helped geladas navigate their large, sprawling communities, which can be made up of as many as 1,000 individuals. Second, while baboons might mate with females for a few days at a time before going their separate ways, gelada males mate with the same group of females for years. It may be that richer, more varied modes of communication helped the sexes stay in more committed relationships.

To find out, Bergman will return to the Simiens next year, where he’ll play back recorded sounds to individual geladas and gauge their responses. There, he’ll try to answer the question: do these wobbles actually transfer information—a hallmark of human speech—or are they merely sweet murmured nothings?